Many solutions are known for achromatic phase retarders, of which only those based on prismatic glass bodies are considered in the following, since the path difference of other solutions (foil combinations, crystal combinations) is significantly more dependent on the wave length.
With a known solution, the FRESNEL rhombus (Appl. Opt. 9 (1970) 9, pages 2123-2129), the preferably desired phase difference of 90.degree., corresponding to a path difference of lambda/4, between the p- and s-components of the light wave that is linearly polarized at an angle to the incident plane, is achieved by double total reflection, the incident light beam striking the retarder entry surface perpendicularly. It is a disadvantage of this solution that the wavelength error of the phase difference in the wavelength range from 400 nm to 1000 nm is .+-.1.3.degree.. In another known solution, a modified DOVE prism (U.S. Pat. No. 4,514,047) three total reflection surfaces are provided. Compared to the FRESNEL rhombus, this has solution has advantages, but still has the same wavelength error. To reduce this wavelength error, glass would have to be used with an index of refraction and with a dispersion, which do not exist in practice.
Furthermore, a solution is known in which a glass prism coated with magnesium fluoride is used, in which four total reflections take place (FILINSKI and SKETTRUP, Appl. Opt. 23 (1984) 16, p. 2747). In the spectral range of 400 nm to 700 nm, the wavelength error of the phase difference is reduced to .+-.0.61.degree.. It is a disadvantage of this method that applying a magnesium fluoride layer on the glass body is technologically complicated and difficult to control.
With another known solution, the hypotenuse of a glass prism is coated with a dielectric multilayer (U.S. Pat. No. 4,595,261). Admittedly, a wavelength error of the phase difference of 0.02.degree. is attained, but only at wavelengths ranging from 740 nm to 980 nm. Here too, another disadvantage is that manufacture of this prism is technologically complicated.